Document the --prefix option to ls-projects

The ls-projects SSH command supports the ability to restrict
the project list by prefix. Document the option in the related
section.

Change-Id: I9adf064eac3c57f3e44b1338452add622785b6c4
diff --git a/Documentation/cmd-ls-projects.txt b/Documentation/cmd-ls-projects.txt
index 486ca44..c5d73ca 100644
--- a/Documentation/cmd-ls-projects.txt
+++ b/Documentation/cmd-ls-projects.txt
@@ -14,6 +14,7 @@
   [--format {text | json | json_compact}]
   [--all]
   [--limit <N>]
+  [--prefix | -p <prefix>]
   [--has-acl-for GROUP]
 --
 
@@ -87,6 +88,9 @@
 --limit::
 	Cap the number of results to the first N matches.
 
+--prefix::
+	Limit the results to those projects that start with the specified prefix.
+
 --has-acl-for::
 	Display only projects on which access rights for this group are
 	directly assigned. Projects which only inherit access rights for